This invention relates to a method for preparing polyamide precursors and poly(amide/benzazole) intermediates for benzazole polymers, to a method for preparing polybenzazole polymers; and to the polybenzazole polymer precursors, poly(amide/benzazole) intermediates and the resulting polybenzazole polymers.
Polymeric materials which are relatively easy to process and useful in ultra high temperature applications such as 300.degree. C. to 400.degree. C. are of interest for preparing films, coatings, fibers, electronic components and composites. Depending on the specific end-use application, the polymers should exhibit one or more of the following properties: mechanical integrity, low moisture absorption, thermo-oxidative stability, thermal stability, solvent resistance, hydrolytic stability, resistance to highly acidic or basic solutions, a low coefficient of thermal expansion and a low dielectric constant. For example, in the electronics industry, it is generally desirable for the material to exhibit a relatively low dielectric constant, high thermo-oxidative stability, good solvent resistance, low moisture uptake and a low coefficient of thermal expansion.
In addition to the mentioned physical properties, the processability of the materials is an important property. For example, in the preparation of electronic components such as wafers and flexible circuit boards, the electronic component is often coated with an insulative or protective coating. To effectively coat the component, the electrically insulative or protective coating is commonly applied from solution and the solvent evaporated.
Currently, polyimide resins are one class of materials which are commonly employed for preparing high strength films, fibers, composites, and coatings, including insulative or protective coatings in the electronics industry. However, polyimide resins suffer from water absorption and hydrolysis which can lead to circuit corrosion and migration of metal ions into the dielectric polyimide layer.
Polybenzazoles (PBX) resins such as polybenzoxazole (PBO), polybenzothiazole (PBT) and polybenzimidazole (PBI) resins exhibit relatively better thermo-oxidative stabilities than polyimides and have improved resistance to water absorption and hydrolysis.
A common method for preparing PBX polymers comprises polymerizing the monomers, commonly an ortho substituted diamine such as bis(ortho-hydroxyamine), bis(ortho-thioloamine) or a tetraamine with an aromatic diacid chloride, dicarboxylic acid or alkyl or aryl diester in a non-oxidizing acid liquid reaction medium, preferably polyphosphoric acid or methanesulfonic acid. The monomers are completely reacted and directly converted to the final PBX polymer under a non-oxidizing atmosphere with vigorous mixing and high shear. The water formed during the reaction, which stops or impedes the polymerization reaction, is compensated by the dehydrating nature of the acid. While the polymer forms a solution in the acid (commonly called a "dope"); it is typically extremely viscous and flows only at elevated temperatures.
The PBX materials can be spun into fibers or extruded into films from their acidic solutions. For example, fibers and films can be directly formed from the polymerization solution ("dope") by extruding or spinning with subsequent removal of the acid solvent. However, the corrosive nature of the acid solvent renders the PBX polymers unsuitable for certain end use applications such as the preparation of electronic components. The polymer can be isolated by evaporation of the solvent or polymer precipitation from the dope and the polymer redissolved in another solvent. While some PBX polymers are soluble in cresol, the preferred solvents are again non-oxidizing acids, including polyphosphoric acid, methanesulfonic acid or sulfuric acid. Therefore, the problems associated with the corrosive nature of the acid solvent are not alleviated. See, for example, U.S. Pat. Nos. 4,533,693; 4,608,427; 4,772,678; and 4,847,530.
There are a number of other methods for preparing PBO polymers. For example, U.S. Pat. No. 3,316,213 teaches preparing aromatic polyamides and aromatic polyoxazoles including PBO polymers by the reaction of a diamine having a structural formula: ##STR1## wherein X is for example hydrogen or alkyl and R is an aromatic tetravalent radical with a diphenyl ester of a dicarboxylic acid having the formula: ##STR2## wherein R.sup.1 is arylene and Y is aryloxy, preferably phenoxy to form a prepolymer such as represented by the formula: ##STR3## Unfortunately, the prepared prepolymer precipitates from solution at relatively low inherent viscosities or degrees of polymerization (i.e., relatively low molecular weights). To process the prepolymer it is melted into desired shape and then converted to the desired PBO polymer. U.S. Pat. No. 3,449,296 teaches the reaction of the same diamine compounds with a diacid halide.
Other techniques for preparing PBO materials from a diamine and a dicarboxylic acid or an acid halide or ester thereof are also described in U.S. Pat. Nos. 4,939,215; 3,408,336; and 4,452,972.
U.S. Pat. No. 3,551,389 teaches a method for preparing aromatic polybenzimidazoles (PBI) by the reaction of an aromatic tetraamine with a diphenyl ester of an aromatic dicarboxylic acid, a diphenyl ester of a heterocyclic dicarboxylic acid or an anhydride of an aromatic dicarboxylic acid. See also U.S. Pat. No. 3,509,108. Again, the described method makes PBI materials which are solid and insoluble in most common organic solvents.
U.S. Pat. No. 4,332,883 teaches preparing a polyoxazole precursor of an olefinic unsaturated monoepoxide on a hydroxy group containing the polycondensation product of an aromatic and/or heterocyclic dihydroxy diamino compound in a dicarboxylic acid chloride or ester. The product is a photosensitive PBO precursor. The prepolymers are prepared by reacting the diamine with an ester or chloride of a dicarboxylic acid at lower than ambient temperatures, e.g., -5.degree. C. to -20.degree. C. in an organic solvent. The resulting prepolymer is soluble in the solvent as a viscous solution, and is precipitated from solution and then reacted with an epoxy such as a glycidylmethacrylate.
U.S. Pat. No. 4,945,153 discloses a method for producing a prepolymer of an aromatic polybenzothiazole (PBT) or polythiazole from an aromatic diaminodithio compound salt in a dicarboxylic acid derivative in the presence of a phosphoric acid compound. The dicarboxylic acid derivatives include those having carboxylic acid groups substituted as follows: ##STR4## wherein Ar is an aromatic ring. The reaction is conducted in the presence of a polyphosphoric acid compound at a controlled temperature such that a prepolymer having a large polymerization degree can be obtained without ring closure. Unfortunately, polyphosphoric acid and its attendant disadvantages are not eliminated in the described method.
In view of the stated deficiencies of the prior art, it remains desirable to provide a method to prepare PBX polymers and oligomers or precursors thereof which are more easily processed as a non-corrosive solution while retaining the excellent physical and chemical properties of the resulting polymer.